I watched train first and then the ghost detective. I am surprised that no one thought the two were similar!
1- the lovestory itself. We know the leads love each other but the show does not make them state the obvious. As in they never declare their feelings to one another but we viewers know how much they love each other. In this way the romance is subtle and beautiful. It touches you deeply especially when you see how far the leads are willing to go for each other.
2- in both shows the leads overcome the impossible to be with one another. In train the ML finds the FL in another universe after her death and in the ghost detective the ML comes back as a ghost to the FL (none of these are spoilers)
3- the antagonists were similar too. Their origin stories, their descent to darkness was kinda similar to me and how it all came down to this question they kept asking

If you enjoyed one you will certainly enjoy the other. I know i did

I felt like the male lead from my mister and the female lead lead from liar game were pretty similar in the sense that:
-both were kind
-both were seen as weak due to their kind natures
-both proved in the end that being kind is a strength despite most people telling them that they are being stupid

The female lead from my mister and the male lead from liar game were similar:
-both have gone through a lot of pain which has made them cold and uncaring to others
-both get frustrated with the kind nature of the other lead
-both protect the leads at all costs
-both are incredibly intelligent

In both dramas, towards the end the leads find out that they had been deceived/lied to by the other lead but don't jump to misunderstandings conflict. They choose to keep trusting the other person because they know the other person.
There's also a huge debt to be paid by the female leads in both dramas.
There's no blatant romance in both dramas. Its left to the viewer's interpretation (and I choose to believe that the leads ended up together in both dramas)